Table 19.1Dielectric Constants and Dielectric Strengths for Various Materials
at 20ºCMaterial Dielectric constantκ Dielectric strength (V/m)
Vacuum 1.00000 —Air 1.00059 3×10^6
Bakelite 4.9 24×10^6
Fused quartz 3.78 8 ×10^6
Neoprene rubber 6.7 12×10^6
Nylon 3.4 14×10^6
Paper 3.7 16×10^6
Polystyrene 2.56 24×10^6
Pyrex glass 5.6 14×10^6
Silicon oil 2.5 15×10^6
Strontium titanate 233 8×10^6
Teflon 2.1 60×10^6
Water 80 —Note also that the dielectric constant for air is very close to 1, so that air-filled capacitors act much like those with vacuum between their platesexcept
that the air can become conductive if the electric field strength becomes too great. (Recall thatE=V/dfor a parallel plate capacitor.) Also shown
inTable 19.1are maximum electric field strengths in V/m, calleddielectric strengths, for several materials. These are the fields above which the
material begins to break down and conduct. The dielectric strength imposes a limit on the voltage that can be applied for a given plate separation. For
instance, inExample 19.8, the separation is 1.00 mm, and so the voltage limit for air is
V = E⋅d (19.58)
= (3×10^6 V/m)(1.00×10 −3m)
= 3000 V.
However, the limit for a 1.00 mm separation filled with Teflon is 60,000 V, since the dielectric strength of Teflon is60×10^6 V/m. So the same
capacitor filled with Teflon has a greater capacitance and can be subjected to a much greater voltage. Using the capacitance we calculated in the
above example for the air-filled parallel plate capacitor, we find that the Teflon-filled capacitor can store a maximum charge of
Q = CV (19.59)
= κCairV
= (2.1)(8.85 nF)(6.0×10^4 V)
= 1.1 mC.
This is 42 times the charge of the same air-filled capacitor.
Dielectric Strength
The maximum electric field strength above which an insulating material begins to break down and conduct is called its dielectric strength.Microscopically, how does a dielectric increase capacitance? Polarization of the insulator is responsible. The more easily it is polarized, the greater its
dielectric constantκ. Water, for example, is apolar moleculebecause one end of the molecule has a slight positive charge and the other end has a
slight negative charge. The polarity of water causes it to have a relatively large dielectric constant of 80. The effect of polarization can be best
explained in terms of the characteristics of the Coulomb force.Figure 19.17shows the separation of charge schematically in the molecules of a
dielectric material placed between the charged plates of a capacitor. The Coulomb force between the closest ends of the molecules and the charge
on the plates is attractive and very strong, since they are very close together. This attracts more charge onto the plates than if the space were empty
and the opposite charges were a distancedaway.
CHAPTER 19 | ELECTRIC POTENTIAL AND ELECTRIC FIELD 681